the snails should not die from the crushed coral, is it possible the acclimation process was not slow enough?
the little hard thing is the snails protective thingy...when they retreat into their shell they close that little trap door over themselves
i dont think there is anything you can do, assuming your water parameters all check out
as far as the crabs, im glad they are doing their thing...hopefully you provided them some empties to move into over time
did you get the skimmer issue resolved?